Retaining walls are used in various landscaping projects and are available in a wide variety of styles. Numerous methods and materials exist for the construction of retaining walls. Such methods include the use of natural stone, poured concrete, precast panels, masonry, and landscape timbers or railroad ties.
In recent years, segmental concrete retaining wall units, which are dry stacked (i.e., built without the use of mortar), have become widely accepted in the construction of retaining walls. An example of such a unit is described in U.S. Pat. No. Re 34,314, which issued to Forsberg (Forsberg '314). Such retaining wall units have gained popularity because they are mass produced and, consequently, relatively inexpensive. They are structurally sound, easy and relatively inexpensive to install, and couple the durability of concrete with the attractiveness of various architectural finishes. The retaining wall system described in Forsberg '314 has been particularly successful because of its use of a block design that includes, among other design elements, a unique pinning system that interlocks and aligns the retaining wall units, thereby providing structural strength and allowing efficient installation. This system is advantageous in the construction of larger walls, when combined with the use of geogrids hooked over the pins, as described in U.S. Pat. No. 4,914,876 to Forsberg ('876).
The shape of the block is also an important feature during installation of a retaining wall. Forsberg '876 illustrates a fairly complex shape for a retaining wall block which is particularly advantageous in the construction of curved walls. The block is symmetrical about a vertical plane which bisects the block at a midway point through the front and back faces.
Many commercially available blocks are symmetrical about a plane bisecting the front and back surfaces. Typically such blocks have planes rather than axes of symmetry, as there are differences between the top and bottom surfaces of such blocks. Clearly, blocks that are substantially square or rectangular (i.e., each surface being joined to another at an orthogonal angle) exhibit a great deal of symmetry. Other blocks are more complex in shape and exhibit only one vertical plane of symmetry. For example, U.S. Pat. No. 5,711,130 (Shatley) illustrates a block having substantially parallel front and back faces and non-parallel, mirror-image side wall surfaces. That is, there is a mirror plane of symmetry that vertically bisects the block. U.S. Pat. No. 5,294,216 (Sievert) illustrates a type of block having parallel front and back faces and non-parallel, converging side surfaces. The term "converging side surfaces" means that the sides walls of the blocks converge as they approach the rear of the block. Such blocks are also symmetrical about a vertical plane that passes through the front and back surfaces.
There are advantages to having non-parallel surfaces on these blocks when constructing a retaining wall. The angles formed by these side surfaces permits construction of curvilinear walls, and moreover, permit the amount of curvature to vary according to the terrain and desired appearance of the retaining wall.
Another important feature of retaining wall blocks is the appearance of the block. Blocks having angled or curved faces are well known in the art. Many manufacturers also vary the color and the texture or pattern on the front face of the block. It might be desirable for the face of the block to be smooth, serrated, or grooved or to have an aggregate appearance.
The look of weathered natural stone is very appealing for retaining walls. There are several methods in the art to produce concrete retaining wall blocks having an appearance that to varying degrees mimics the look of natural stone. One well known method is to split the block during the manufacturing process so that the front face of the block has a fractured concrete surface that looks like a natural split rock. This is done by forming a slab in a mold and providing one or more grooves in the slab to function as one or more splitting planes. The slab is then split apart to form two or more blocks.
Another method to create a weathered stone appearance is to tumble the blocks together with other blocks in a large rotating canister. The collisions of the blocks in the tumbler chips off random pieces of the blocks, rounding the edges and creating a look that can be quite close to the appearance of a natural stone. This is a labor intensive undertaking that also can result in undesirable damage to the blocks and high overall costs of production.
Another method to make naturally appearing blocks has been described in U.S. Pat. Nos. 5,078,940 and 5,217,630 (both to Sayles). These patents describe a method and an apparatus for manufacturing a concrete block having an irregular surface. The irregular surface can be made to look similar to split stone, and thus is very desirable. This is done by pouring uncured block material into a mold cavity and causing a portion of the material to be retained in place relative to the cavity walls when the block is removed from the cavity. This results in a split appearance for the surface, without having to perform the splitting operation. This is an advantage because the expense and time of conventional block splitting is avoided.
Typically, retaining wall blocks are manufactured to have the desired appearance on the front face (i.e., the outer face of a wall) only. In the patents described above, the pattern or design is typically provided only to the front face because that is the only portion of the retaining wall block that is visible after the wall is constructed. Sometimes a portion of a side surface may be provided with a desired pattern or texture. Further, sometimes two opposing surfaces of a cap block may be textured. In the Sayles patents described above, a natural or split look is obtained for only the front face. Such blocks do not allow the user the option to use either the front, side, or back faces of the block interchangeablity as the exposed "front face". To create a wall block that has a roughened texture on the front, side and back surfaces poses certain problems. If a splitting method is used, multiple splits and two orientations for the splits are required to create a quadrilateral block with texture on three sides. If a tumbling method is used, substantial portions of the block faces will be smooth and not entirely natural looking. Tumbling also is an expensive production method. If the method combines splitting and tumbling, the cost of production increases to a point where the end cost to the consumer is very high.
Creating a random, or ashlar, pattern in the face of a retaining wall is highly desirable. This gives the appearance of a mortared or dry-stacked natural stone wall, which is a traditional and well accepted look. Some current wall blocks are intended to create an ashlar pattern. However, the creation of a truly random appearance requires the production of multiple block shapes for use in a single retaining wall. This is inefficient from a production standpoint because this requires multiple molds and more kinds of blocks to inventory. If only one face of the block is intended to be the front face, then the block system will suffer a trade-off between having enough face sizes to create a random, natural appearance and the cost and inefficiency of using multiple molds and creating multiple inventory items.
Because of the natural variation in size of the stones used in stone retaining walls, the wall surface has variations in depth from stone to stone. None of the prior art concrete segmental retaining wall blocks is capable of duplicating this effect due to their alignment and connection systems requiring uniform alignment of the blocks and their front faces. It would be desirable to produce a block that could have random variations in face depth while maintaining the structural integrity of the wall structure.
It would be desirable to provide a system of blocks for constructing a retaining wall that combines the ease of installation of modern segmental retaining walls with the attractive appearance of a natural stone wall composed of stones of varying sizes. The block system should be efficient to produce, require a minimal number of different block shapes and allow the construction of walls with 90 degree corners, and the construction of freestanding walls with a desirable natural appearance. It would also be desirable to provide a retaining wall system that allowed an aesthetically pleasing randomness of appearance by varying the amount the front faces of individual blocks project out from the face of the wall, so that certain blocks project slightly out and others are slightly recessed, at the wall builder's option. Moreover, it would be desirable to provide a retaining wall block with a weathered appearance on at least three sides that could be manufactured in a manner that minimizes the need for splitting or tumbling the block.